Justin B. Hensley, Eric P. Webster, Dustin L. Harrell and Sunny L. Bottoms

In 2007, Louisiana producers planted 605,000 acres into soybeans. Approximately 95 percent of those acres were planted in Roundup Ready varieties, which are resistant to the herbicide glyphosate. That same year, approximately 26 percent of Louisiana’s 348,000 rice acres were planted in Clearfield varieties, which are resistant to the herbicides imazethapyr (Newpath) and imazamox (Beyond). In addition, 21 percent of the rice acreage was ratooned – harvested a second time – from the stubble that regrew after initial harvest. These fields of Roundup Ready soybeans and Clearfield rice are often grown adjacent to fields of rice varieties susceptible to the herbicides used in these cropping systems. This creates a great potential for damage to rice from the off-target movement of these herbicides.
 
In recent years, researchers at the LSU AgCenter Rice Research Station near Crowley have evaluated the effects of simulated herbicide drift into rice fields. Two major objectives of this research are to quantify the potential yield losses caused by herbicide drift in both the primary and ratoon rice crops and to better understand the visual symptoms of injury caused by a particular herbicide.
 
Two studies evaluated drift of Roundup and Newpath to rice. These two herbicides account for a vast majority of all reported off-target herbicide drift. Two reduced rates of each herbicide were applied at one of four growth stages – one-tiller, panicle differentiation, boot and harvest maturity. The one-tiller treatment was applied to rice when plants were beginning to tiller – or produce sprouts. The panicle differentiation treatment was applied at the onset of reproductive growth when the panicle – or flower cluster – first began developing. The boot treatment was applied to rice in the late reproductive phase of growth immediately prior to panicle emergence. The harvest-maturity application was done when the rice was at approximately 20 percent grain moisture within a week of harvest.
 
Both herbicides reduced primary crop yield when applied at the one-tiller, panicle differentiation and boot stages (Figures 1 and 2). Ratoon crop yield was reduced by applications at one-tiller and panicle differentiation but was increased at the boot stage. This increase in ratoon crop yield was attributed to excessive tillering that did not produce harvestable seeds in the primary crop but did in the ratoon crop. When the primary and ratoon crop yields are combined to calculate total yield, however, overall yield was reduced compared with the nontreated control. Simulated herbicide drift applied to rice at harvest maturity had no effect on the primary or ratoon crops.
 
When using visual symptoms to evaluate the possibility of drift in a field, it is important to look at the big picture before trying to associate the symptoms with a particular herbicide. In the case of drift, distinguishable patterns usually occur, and many other factors must be considered to differentiate herbicide drift from injury caused by disease or insects. One factor to look for is if damage is more severe on one side of the field than the other. Another factor is if the vegetation at the field edge and in buffer areas between the injured field and an adjacent field has symptoms indicating herbicide drift. Basically, if drift has occurred, the herbicide has to have come from an application to a target field and will usually leave a noticeable path as it moves off target. These patterns are often observed on a side of the field associated with the predominant wind direction. Levees often can be used to determine the direction of drift. In many cases, symptoms will occur only on one side of the levee.
 
If a pattern can be established and the factors evaluated indicate the presence of herbicide drift, then the symptoms can be associated with a particular herbicide. The two most common herbicides involved in drift in Louisiana are Roundup and Newpath. Research has shown that it is easier to distinguish drift of Newpath from Roundup when it occurs to rice in the vegetative growth stages (from emergence through tillering). Both herbicides will cause the newly emerging leaf to be tightly rolled and yellowing. In the case of Roundup drift, often the tips of older leaves will have a "burned" appearance. Some symptoms specific to Newpath drift are yellowing of the new leaf being restricted to the leaf area between the veins. If plants recover, they often will tiller along a single plane, developing a flat-fan appearance. Plants affected by Newpath also will have leaf lesions similar in appearance to the lesions from leaf blast disease. These symptoms do not occur on plants affected by Roundup drift.
 
For drift occurring during the reproductive growth stages of rice (from panicle differentiation to boot stage), the symptoms from both herbicides are very similar. It is common to see a shortened or distorted flag leaf (the first leaf below the panicle) and malformed and blanked or unfilled grains on the panicle. For drift occurring during the boot stage, symptoms are excessive tillering from the upper nodes of the plant and failure of the panicle to fully emerge from the boot or sometimes not to emerge at all. This lack of panicle emergence is more commonly observed from Newpath drift compared with drift from Roundup. In severe cases, it can cause death of the flag leaf.
 
Determining if herbicide drift has occurred is not an exact science. No single factor can lead one to determine if herbicide drift has occurred; it is more of a combination of several factors determined from observation. The main point about herbicide drift on rice is that is can be detrimental if it occurs at any time during the growing season, so applicators should be aware of adjacent crops, wind direction and wind speed when applying herbicides near susceptible rice fields.